1. Field of the Invention
This invention relates to a multipath transmission system for transmitting data among multiplex nodes connected to common signal transmission lines.
2. Description of the Related Art
This type of multipath transmission system includes a transmission system of LAN (Local Area Network) utilizing a CSMA/CD (Carrier Senser Multiple Access/Collision Detection) with an NDA (Non Destructive Arbitration). A typical example of the LAN is a CAN (Controller Area Network) used for data transmission in cars, for example.
In the multipath transmission system utilizing the CAN, terminal resistors R.sub.E, R.sub.E are connected to both ends of two signal transmission lines A and B and a plurality of multiplex nodes are connected in parallel with the signal transmission lines A and B.
Each of the multiplex nodes is constructed by a communication control device, a transmission circuit for receiving a transmission signal from the communication control device and transmitting the same to the signal transmission line, and a reception circuit for receiving a signal from the signal transmission line and transmitting the same to the communication control device.
The reception circuit outputs a signal received from the signal transmission line to a comparator of the communication control device. An adequate threshold voltage is applied to the comparator to reduce the amplitude of the transmission signal to a permissible level so that the range of an input voltage in the common mode of the reception circuit can be widen and influence by noises can be reduced.
The transmission circuit includes two field effect transistors (FET) and the FETs are connected to the signal transmission lines.
Therefore, when the multiplex nodes are set in the passive state, that is, when a recessive bit is output in the transmission system of (CSMA/CD+NDA), each FET is set in the OFF state, no potential difference occurs between the signal transmission lines and a high impedance state is set the output terminal. Further, when the multiplex nodes are set in the dominant state, that is, when a dominant bit is output in the transmission system of (CSMA/CD+NDA), each FET is set in the ON state, a current is supplied to one of the transmission lines and a current is drawn from the other signal transmission line. Therefore, in the dominant state, a potential difference occurs between the signal transmission lines and the reception circuit connected to the signal transmission line detects the potential difference and converts this signal in order to recognize the signal face. As a result, the communication control device can detect the dominant state.
With the above transmission system, when an accident such as an open circuit or short circuit occurs in one of the signal transmission lines, for example, and thus when fault occurs in the transmission system, a potential between the signal transmission lines is changed from a transmissible value so that a signal cannot be correctly transmitted between all of the multiplex nodes associated with the signal transmission line.